This disclosure relates generally to diagnostic imaging and, more particularly, to an apparatus and method of optimizing a scanning protocol to minimize dose.
Typically, in computed tomography (CT) imaging systems, an x-ray source emits a fan or cone-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image.
Generally, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom. Typically, each scintillator of a scintillator array converts x-rays to light energy. Each scintillator discharges light energy to a photodiode adjacent thereto. Each photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are transmitted to the data processing system for image reconstruction. Imaging data may be obtained using x-rays that are generated at a single polychromatic energy. However, some systems may obtain multi-energy images that provide additional information for generating images.
During scanning to acquire projection data, it is generally desirable to reduce x-ray dose received by the subject, thus protocols have been developed that reduce x-ray tube power and patient exposure during image data acquisition. Also, gantry speeds in CT imaging generally continue to increase over time, in an effort to capture images in a shorter time period to reduce motion artifacts. Thus, as x-ray tube power is reduced, and gantry speed is increased, the signal itself may correspondingly decrease, which itself can lead to a lower signal-to-noise ratio (SNR). Thus, protocols have been developed to improve data acquisition and image reconstruction based on data having decreased SNR.
For instance, the concept of dose modulation in CT has been used extensively by each of the manufactures in the form of tube current modulation as a function of table position and x-ray tube angle. In both cases the aim is to deliver an ideal tube current such that the image noise remains relatively constant throughout the volume even as the path length for the x-ray at different positions changes. The parameters for the modulation are typically based on assuming the patient is composed of water cylinders, which are estimated from the scout (i.e., single projection) images. While this approach may offer significant advantages compared with un-modulated acquisitions, there is additional optimization in the x-ray dose modulation which could improve image quality or reduce dose. And, in addition to the tube current modulation, other methods for dose modulation include but are not limited to: kVp modulation, aggressive static bowtie filters, dynamic pre-patient collimation, and dynamically pulsing the x-ray source, as examples. Each of these methods provides a different mechanism to modulate the dose delivered through-out the examination.
However, the above techniques, though known, may not result in a protocol that is optimized for a given patient or for a given scanning scenario. More particularly, although scanning protocols are generally known, selected protocols may not account for particulars of a given scenario. That is, a selected or known protocol may not account for patient size, anatomy, or position, as examples. Further compounding protocol selection, a selected or known protocol may not account for a particular task of the scan (i.e., head CT Angiography, lung, or kidney stones). In other words, scanning protocols are generally known, but typically do not take into account specific patient parameters or particulars of a task that is being performed.
Therefore, it would be desirable to optimize a scanning protocol to minimize dose, based on specifics of the patient and the conditions under which the scanning protocol are implemented.